As a conventional brake device such as a disk brake, for example, Patent Literature 1 discloses friction surface coating for a friction material of a brake device, in which a pad friction flat-surface of a brake pad is pressed against the disk friction flat-surface of a disk rotor rotating together with a wheel, and braking force is applied to the wheel by virtue of frictional force between the disk friction flat-surface and pad friction flat-surface according to the sliding of the one flat-surface against the other.